The present invention relates to a method for making a 4-hydroxy-2,2,6,6-tetraorgano-1-piperidinyloxy, which can be employed as an anaerobic gelation inhibitor during the stripping step in the production of silicon containing polyacrylate hardcoat compositions.
As shown by Chung, U.S. Pat. No. 4,478,876, a process is provided for applying a UV curable silicon containing polyacrylate hardcoat composition onto a substrate, such as a thermoplastic substrate. The UV cure of the Chung polyacrylate hardcoat composition is effected in a non-inert atmosphere, such as air.
In the preparation of a UV curable silicon containing polyacrylate hardcoat composition, an aqueous colloidal silica is initially treated with an alkoxysilylacrylate such as methylacryloxypropyltrimethoxysilane. Water is then removed from the mixture in the form of an aqueous/organic solvent azeotrope. Prior to stripping, a polyfunctional reactive acrylic monomer, such as hexanedioldiacrylate, can be added to the mixture to provide an acrylic matrix for the silylated colloidal silica. Experience has shown that gelation of the silicon containing polyacrylate hardcoat mixture can occur readily during the distillation step, unless an inhibitor, such as methylhydroquinone (MEHQ) is used. As taught by Kurland, J. Poly. Sci. Poly Chem. Ed., 18 (1980) 1139, inhibitors such as MEHQ require oxygen to be effective. As a result, safety concerns have arisen concerning the distillation of large volumes of organic solvent at elevated temperatures in the presence of air or oxygen.
As shown in copending application RD-23,492 filed concurrently herewith, anaerobic gelation inhibitors have been found useful for making silicon containing polyacrylate hardcoats in a non-oxidizing atmosphere. Particularly useful anaerobic gelation inhibitors are the free radical derivatives of tetraorganopiperdinyloxy compounds such as 4-hydroxytetramethylpiperdinyloxy (4-OH TEMPO) having the formula, ##STR1## where R is a C.sub.(1-8) alkyl radical and preferably methyl.
However, due to the limited availability of source materials, for tetraorganopiperdinyloxy compounds, a satisfactory synthesis of 4-OH TEMPO has not been economically feasible.
It would be desirable therefore to be able to provide an economic procedure for synthesizing anaerobic gelation inhibitors, such as 4-OH TEMPO, to satisfy the need for replacing anaerobic inhibitors such as MEHQ during the production of silicon containing polyacrylate hardcoat compositions.